The present invention relates to a detector for indicating reception disturbances during ultrashort wave frequency modulated broadcast reception. Such detectors are used, for example, for the improvement of broadcast reception in power vehicles. The object of the detector is to recognize and indicate a reception disturbance. For this purpose a shift operation is introduced which is generally carried out electronically. For example, the Offenlegungschrift DE-A1 3,107,970 describes an FM receiver in which the multipath noise is avoided with the aid of a detector and a shifting device. In the Offenlegungschrift DE-A1 No. 3,122,057 a tuner control with a detector for detecting the signal level is provided in a broadcast receiver. Moreover, the U.S. Pat. No. 3,825,697 discloses a shifting device which, after recognition of the disturbance because of the multipath reception, shifts from stereo operation to mono operation. In all of the above described cases a detector for recognition of the disturbance is required.
A detector for recognition of disturbances is also known from the U.S. Pat. No. 4,216,353. This detector is especially designed for recognition of disturbing multipath propagation of electromagnetic waves with greatly different transit times. As a result of this effect, an increased noise and a distortion of the low frequency communication takes place at the output of the frequency demodulator. In the case of stereo transmission this fact also results in an increased cross-talk between both stereo channels. The detector described in this patent is based on the evaluation of the amplitude-time behavior of the frequency demodulated signal available in the receiver. This detector has, in case of its use in an automobile receiver, the following disadvantages. It is known that superposition of partial waves at the reception point in the event of transit time differences between 1 .mu.s and 100 .mu.s leads to significant distortions of the low frequency communication at the output of the FM of an FM receiver. This distortion passes, together with an amplitude modulation of the resulting high frequency carrier that depends, on the low frequency communication content, to the reception point. The detector disclosed in the U.S. Pat. No. 4,216,353 recognizes these amplitude modulations and indicates them as disturbances. However, the wave field is in most instances made up of partial waves whose transit time differences lie below 1 .mu.s. This superposition of the partial waves does not lead to disturbances at the reception point, but causes a strong dependence of the resulting field amplitude on the reception location. The input level of the automobile receiver experiences, as a result of the movement of the vehicle in this wave field, a time-dependent amplitude change which appears as an amplitude modulation. A disadvantage of the above described detector is that it must distinguish between an amplitude modulation type which does not lead to disturbances and another amplitude modulation type which results from multipath propagation with large transit time differences and, thereefore, causes disturbances. This situation is especially difficult because both modulation types take place statistically and chronologically in a simultaneous manner. This results in an unreliable recognition of the actual disturbance and a relatively long detection time. This long detection time leads to a situation where the tuning in the reception system by switching can be carried out only so late that the broadcast listener has already sensed the disturbance.